Overview Our research spans a range of topics in biothermal and building energy efficiency. Both modeling and experiments are used to study a variety of fundamental heat and mass transfer processes for improved fundamental understanding and technology design.

BUILDING ENERGY EFFICIENCY

1. Building energy simulations a

(a) Three-dimensional geometry of a generic slab block building in Singapore; and (b) demonstrating cooling energy savings based on the developed ETTV equation for different façade types.

Energy consumption of buildings takes up about a third of Singapore’s total electricity production. This work intends to address key issues related to facade and envelope designs, and their impacts on buildings’ air-conditioning performance in tropical climates. Key issues addressed in this work include: (1) determining, through computer simulations, the ETTV (Envelope Thermal Transfer Value) equation and its respective coefficients for residential/commercial buildings; (2) investigating the effects of building aspect ratio and orientation on heat gain for residential/commercial buildings; and (3) to study the extent of influence shading effects due to opaque walls and corridors has on buildings’ heat gain. This work investigates the energy performance of buildings with varying facade design. Beginning with an energy survey of households, we established the building’s air-conditioning usage patterns and modelled the impact that various facade design has on building heat load. Subsequently, ETTV equations were fundamentally developed both residential/commercial buildings. In addition, a set of energy load estimating equations has also been developed using computer simulations and local climatic data. These equations provided an accurate predictive means of estimating the annual cooling energy consumption of buildings both residential and commercial. A first-of-its-kinds study was also conducted to investigate the impact of weather conditions on the developed methodology for estimating the energy consumption of buildings.

We pioneered a design-day weather file to provide simplicity, flexibility and greater ease of use. The design day concept is pivotal in providing key inputs to the cooling energy-estimating methodology yielding good agreement with DOE-2.1E simulated results. Without a doubt, the refined method has benefited building authorities in their pursuit of developing stringent building energy standards in order to realize better energy efficient buildings.

2. Evaporative air-conditioning systems

Schematic of the proposed improved dew-point evaporative air cooler based on M-cycle: (a) one-unit channel pair; and (b) plan view.

Our research on new cooling technology focuses on the fundamental development of a novel dew-point evaporative air cooler, depicted in Figure 3.5, is theoretically studied to promote dew-point cooling effectiveness. The novel dew-point evaporative air cooler, based on a counter-flow closed-loop configuration, is able to cool air to temperature below ambient wet bulb temperature and approaching dew-point temperature. A computational model for the cooler has been developed. The model demonstrated close agreement with the experimental findings to within 7.5%. Employing the validated model, we studied the cooler performance due to the effects of (i) varying channel dimensions; (ii) employing room return air as the working fluid; and (iii) installing of physical ribs along the channel length. Using these means, we have demonstrated improved performance of the dew-point cooler enabling it to achieve break-through cooling efficiencies never realized before - wet bulb effectiveness and dew-point effectiveness spanning 122% to 132% and 81% to 93%, respectively. The secret to our cooler performance is to design channels that enable better evaporative to take place by producing better fluid turbulence and air interactive. To achieve higher cooling effectiveness, this work has found that the length of the channel passage should be at least 200 times of the height of the working channel. When the inlet velocity is more than 1.5 m/s, the wet bulb effectiveness would increase by 10 - 20% for a cooler with ribs installed, compared to a purely plain channel.

3. Novel Dehumidification – Hybrid membrane and composite desiccant

(I) Novelty of our membrane technology and (II) the synthesis process and SEM images of the composite ceramic-ploymeric membrane with SEM images.

The synthesized membranes are based on strength of metallic mesh, high diffusivity of ceramic material, and high water vapour selectivity of highly hydrophilic polymers. The materials used are judiciously chosen so that they are cheap, abundant, physically and chemically durable, easy to manufacture, and suitable for up-scale production. Our membranes incorporate the unique characteristic of each component, namely, strength and durability of stainless steel, high diffusivity of ceramic membrane and high selectivity of hydrophilic polymer. Testing results have shown that the membrane dehumidifies air efficiently. It easily turns humid input air with 90% relative humidity into a drier and comfortable output air with 55% relative humidity. This innovative membrane technology was featured as one of most creative technologies developed in Singapore – “Sunday Times September 21, 2014”.

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Commercial solid desiccant wheels require high temperature thermal heat to ensure sustainable operation. Silica-gel, the most effective desiccant for air/gas dehumidification, requires temperature often greater than 120oC. Additionally, large pressure drops across the desiccant wheels translates to higher operating cost due to larger fan power. My work is focused on pushing the boundary of engineering science in order to fundamentally develop the next generation of low-cost noncorrosive low-vapour pressure, and safe composite desiccants with desirable sorption properties to reduce energy use. Thus far, the results obtained from the different types of composite desiccants synthesized in my laboratory have demonstrated higher moisture removal capacity when compared with the commercially prevalent silica gel, as demonstrated It improves the moisture removal capacity by up to 83%. This translates to a more compact moisture adsorbing system resulting in a lower pressure drop across the system.

BIOTHERMAL RESEARCH

1. Experimental in-vitro cryosurgery In exploring ways to cryo-freeze complex irregularly-shaped tumours, we have pioneered a discrete algorithm that enables the placement of multiple freezing probes and suggest freezing protocols that would assist surgeons in determine the optimal cryo-freezing method for irregularly shaped tumours. In other words, to “cryo-sculpt” these tumours while maximising cancer cell destruction. A highly instrumented experimental setup has been developed in my laboratory to perform various in-vitro experiments using bio-simulated gel or porcine tissue such as liver and kidney. Experiments have been performed to validate different ice-shapes formed by different combinations of multiple cryoprobes. Our cryosurgery research achievement includes developing simplified transient cryo-freezing and thawing processes to enhance cell destruction within tumours. Other innovative ideas that I have examined in the area of ice-ball contour control in complex-shaped tumours include multiple freezing/thawing processes. This simple mechanism has demonstrated to be particularly useful in maximising cell death especially along the boundaries of the tumour.

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(a) A highly instrumented biothermal experimental setup to study cryo-freezing of biological samples, and (b) in-vitro experiments conducted using porcine livers and bio-gel (multiple probe freezing).

2. Cryoablation of vascularized solid tumors (I) (II)

(I) Thermographs of porcine liver experiments with no large blood vessel; (b) infrared thermographs for pig liver experiment with a single large blood vessel; and (c) infrared thermographs for pig liver experiment with parallel counter-current vessel pair; and (II)transient temperature development of a vascular liver during cryoablation with blood flow rate at 1000ml/min: (a)temperature contour at 9min;(b)temperature contour at 15min; (c)temperature contour at 25min; and (d)temperature contour at 30min. (Note: Vascular tree and tumour are clinically extracted from the MRI-image of a liver cancer patient)

Anatomically, solid tumours are often situated close to or embedded within large blood vessels in a complex vascular system. The blood flow inside the blood network represents a heat source to the nearby frozen tissue and thereby limits the development of cryo-lesions. Convective effects from other neighbouring blood vessels on temperature distribution in ablation sites have further added to the challenge of achieving adequate cryoablation. Our research focus further includes the development of a computational model that incorporates a simplified thermal-equivalent mathematical description of complex vascular morphology. Complex vascular network with varied blood flows are simplified and modelled as tree-like branched fractal network. The present work evolved a simplified and time-saving methodology to accurately simulate complex blood vessel network in order to reduce simulation tediousness and computational cost. In addition, the developed algorithm generates transient temperature profiles to enable data gathering from the isotherms in the anatomical region of interest, and provides essential information on the ice-front propagation. The objective of this research is to develop a conceptual framework to impact clinicians think through and describe the possible outcomes of their surgical plans particularly in dealing with heavily vascularised solid tumours with irregular boundaries.

3. Thermally enhanced nano-cryosurgery

Experimental data showing the introduction of gold nanoparticles injected into the porcine tissue at approximately 30 mm in the radial direction away from the cryoprobe’s surface, and influence of blood vessel on the freezing front propagation in the radial direction: (a) single and double vessels without nanoparticles; and (b) single and double vessels with gold nanoparticles. The primary purpose of incorporating nanotechnology to cryosurgery is to develop enhanced and better controlled cryo-therapy. The intentional loading of nanoparticles with high thermal conductivity into the tumours can significantly lower the final temperature of the tissue, promotes freezing rates, and sustains ice-front propagation even in the vicinity of major blood vessels. Therefore, incorporating nanoparticles to cryosurgery improves the treatment efficiency of conventional cryosurgery. The key contributions of my nano-cryosurgery work include (1) demonstrating the effectiveness of incorporating different nanoparticles to enhance cryo-freezing, (2) investigating the temperature response of biological tissue subjected to injection of nanoparticles, and (3) understanding the loading of different nanoparticles at critical sites in order to tailor the growth of the ice-ball to the tumour’s shape and to reduce unintended destruction of healthy tissue. In addition, We have demonstrated that, via the use of nanoparticles, the receding of the ice-ball growth due to the heat source effect of large vessels can be prevented. Ice-ball front movement is markedly influenced by the number of key blood vessels. Ice-ball starts to recede beyond a specific distance away from the cryoprobe. The injection of thermal enhancing nanoparticles around the peripheral of the receding point enables the ice front to sustain its march forward towards the boundary of the tumour. Hence, the timely intervention of nano-assisted cryo-freezing is clear and appreciable.